The present invention relates generally to biphasic processes for the polymerization of polymers and more particularly concerns such polymerization processes using pH-sensitive monomers.
The biphasic polymerization of bisphenols with phosgene is a common method for the preparation of polycarbonates. Generally, polycarbonate preparation involves the phosgenation of an aqueous alkaline solution of the bisphenol in the presence of an inert organic solvent and typically an amine catalyst. The pH can be quite high ( greater than 12) when an excess amount of alkaline base is used, or may be controlled between pH 8-10. The pH in all of these cases is used to control the final optical (i.e., color) properties of the material. In all cases, the bisphenol is extremely hydrolytically stable over the entire pH range and molecular weight control is usually achieved by the use of monofunctional end-capping reagents.
U.S. Pat. No. 5,416,185 to Becraft and Ramsey concerns a conventional process for preparation of polycarbonates. In particular, the patent disclosed a method for producing polycarbonates by an interfacial reaction of phosgene and bisphenol in a two-phase reaction medium containing an aqueous hydroxide and an organic solvent such as methylene chloride. According to the patent, phosgene usage in excess of about 15 mole percent above stoichiometrically predicted amounts was eliminated by controlling the pH of the medium to range between 8 and 10 and controlling the amount of water in the reaction medium so that high salt conditions were reached at the end of the phosgenation. A bisphenol specifically exemplified in the patent, bisphenol-A, is hydrolytically stable at high pH.
U.S. Pat. No. 5,198,507 discloses bioerodible polycarbonates prepared from amino acid-derived diphenols disclosed in U.S. Pat. No. 5,099,060. The disclosures of both the ""507 patent and the ""060 pagent are incorporated herein by reference. A particularly useful diphenol monomer disclosed in U.S. Pat. No. 5,099,060 is desaminotyrosyl tyrosine ethyl ester (DTE).
DTE is an extremely pH-sensitive bisphenolic monomer. Attempts to polymerize this bisphenolic monomer with phosgene via a classical biphasic polymerization process resulted in severe monomer hydrolysis, and consequently a failure to synthesize the desired poly(DTE carbonate). This problem exists in general with the diphenol monomers disclosed in U.S. Pat. No. 5,099,060. A need exists for a biphasic process that is suitable for use with pH-sensitive monomers to synthesize polycarbonates, polyesters, polyamides and other polymers that may be prepared by biphasic methods.
It has now been discovered that strict pH control can be applied to biphasic processes, so that pH-sensitive monomers can be polymerzed to form a wide variety of useful polymeric products.
Therefore, according to one aspect of the present invention, in a biphasic polymerization process including the steps of:
admixing an aqueous solution of a first monomer, the first monomer being hydrolytically unstable below a pH of about six or above a pH of about eight, with a water-immiscible organic solvent;
adding to the admixture a catalyst selected from the group consisting of tertiary amine, quaternary amine and phosphonium catalysts, an acid-forming co-monomer for the first monomer and an acid scavenger; and
recovering the resting polymer,
the improvement includes providing the aqueous solution at a pH between about six and about eight; and adding to the admixture the acid-forming co-monomer and the acid scavenger at relative rates effective to maintain the pH of the admixture in a range from about six to less than eight.
According to another aspect of the present invention, in a biphasic polymerization process including the steps of.
admixing an aqueous solution of a first monomer, the first monomer being hydrolytically unstable below a pH of about six or above a pH of about eight, with a water-immiscible organic solvent;
adding to the admixture a catalyst selected form the group consisting of tertiary amine, quaternary amine and phosphonium catalysts, an acid-forming co-monomer for the first monomer and an acid scavenger; and
recovering the resulting polymer,
the improvement includes providing the aqueous solution at a pH between about six and about eight; and adding to the admixture the acid-forming co-monomer and the acid scavenger at relative rates effective to maintain the pH of the admixture between about six and about nine, the molar ratio of acid-forming co-monomer to first monomer being 1.4:1 or greater.
The biphasic polymerization process of the present invention is particularly useful for the polymerization of hydrolytically unstable diols, especially diphenols. For bisphenols, the co-monomer is typically a dihalide selected from: 
wherein X is a halogen, R is carbon or sulfur and Z is an aryl, alkyl, alkylaryl, alkyl ether, aryl ether or alkylaryl ether group containing up to 18 carbon atoms. When the pH-sensitive monomer is a diphenol and the dihalide is phosgene, the resulting polymer is a polycarbonate.
The present invention incorporates the unexpected discovery that the preferred narrow six to eight pH range also permits catalyst control of the final weight average molecular weight. In particular, it has been discovered that a roughly linear relations exists between weight-average and number-average polymer molecular weight and the molar ratio of amine catalysts to first monomers. Because the relationship is roughly linear, it is possible to use catalyst and first monomer concentrations to control polymer molecular weight without undue experimentation.
While not being bound by any particular theory, it is believed that the preferred pH range between about six and about eight permits catalyst control of polymer molecular weight because at specified molar ratios of catalyst to first monomer within this pH range, the catalyst becomes deactivated in the course of the reaction. The extent of the polymerization, and consequently the polymer molecular weight, is thereby controlled by the amount of catalyst relative to the first monomer. Above a pH of about eight, the catalyst is regenerated, and the amount of catalyst cannot as a practical matter be used to control the final polymer molecular weight.
The present invention thus provides a method to control the final weight-average or number-average molecular weights of biphasicly prepared polymers in general, without the use of end-capping reagents, and without controlling reactant stoichiometry. Thus, according to another aspect of the present invention in a biphasic polymerization process including the steps of:
admixing an aqueous solution of a first monomer with a water-immiscible organic solvent;
adding to the admixture a catalyst selected from the group consisting of tertiary anine, quaternary amine and phosphonium catalysts, an acid-forming co-monomer for the first monomer, and an acid scavenger; and
recovering the resulting polymer;
the improvement includes providing the aqueous solution at a pH between about six and about eight, adding the amine catalyst to the admixture at a molar ratio to the first monomer effective to provide a predetermined weight-average or number-average molecular weight for the resulting polymer, and adding to the admixture the acid-forming co-monomer and the acid scavenger at relative rates effective to maintain the pH of the admixture in a range from about six to less than eight.
The ability to use amine catalyst concentration and strict pH control to determine final polymer molecular weight applies to biphasic polymerization monomers in general, and to both monomers that are hydrolytically unstable and monomers that are hydrolytically stable. A biphasic process is provided that makes possible the polymerization of end-functionalized polymers that may be further derivatized.
Other features of the present invention will be pointed out in the following description and claims, which disclose the principles of the invention and the best modes which are presently contemplated for carrying them out.